On both Linux and OS X, you can 'freeze' an app by sending it the old-style unix signal SIGSTOP. I've done this before, and it sort-of-works, but it doesn't ten

On both Linux and OS X, you can 'freeze' an app by sending it the old-style unix signal SIGSTOP. I've done this before, and it sort-of-works, but it doesn't tend to play very well with GUIs. If an app has a window up when you SIGSTOP it, the window will hang until you send the app SIGCONT to continue. And under OS X, at least, if you leave an app stopped for too long it seems to make the rest of the GUI become unresponsive; so I wouldn't really recommend it. But it's something you can experiment with.